The invention relates generally to a method and a plant for processing milk and, in particular, to a method and a plant for producing a sterile milk product.
Milk processing typically involves several steps, including separation of raw milk into a milk portion and a cream portion, filtering the milk portion to form a permeate and a retentate, heating and/or filtering the permeate to reduce the number of live microorganisms, heating the cream to reduce the number of live microorganisms, recombining the treated cream and treated milk, homogenizing the combined milk and cream, and packaging the resultant milk product. The retentate resulting from microfiltration of the milk product can be recycled into the cream or milk processing stream, used in other processing streams, or discarded.
In order to increase the efficiency of milk processing and reduce costs, it is desirable that the amount of raw milk lost or discarded during each step of milk processing be minimized.
Raw milk naturally contains various microorganisms such as bacteria, yeast and mold. Additionally, raw milk may come into contact with and retain additional microorganisms during storage and transport. These microorganisms can contribute to the degradation and eventual spoilage of milk. It is therefore desirable to remove or deactivate these microorganisms.
Microorganisms such as bacteria can be categorized by size and/or heat sensitivity.
Heat resistant bacteria are bacteria which can be deactivated by, for example, heating to a temperature of from at least about 140xc2x0 C. to about 150xc2x0 C. for about 4 to about 6 seconds as known to practitioners in the art. This category of bacteria is normally referred to as spores, and includes bacteria of species Bacillus and Clostridium. These bacteria, as defined by their least diameter, are about 0.5 xcexcm or larger. The smallest heat resistant bacterium identified in milk by the inventor is Bacillus pumilis, which has a least diameter of about 0.5 xcexcm.
Thermoduric bacteria are bacteria which are deactivated by, for example, heating to temperatures of about 100xc2x0 C. for a few seconds, or equivalent time and temperature combinations as known to practitioners in the art. The smallest thermoduric bacterium identified in milk by the inventor is microbacterium Lacticum, which has a least diameter of about 0.3-0.4 xcexcm. Thus, these bacteria have a least diameter of about 0.3 xcexcm or larger.
Heat sensitive bacteria are bacteria which are deactivated by, for example, heating to temperatures as low as about 72xc2x0 C. to about 75xc2x0 C. for about 15 to about 20 seconds, or equivalent time and temperature combinations as known to practitioners in the art. The smallest heat sensitive bacterium has a least diameter of less than about 0.3 xcexcm.
Raw milk also contains naturally occurring enzymes, as well as extracellular enzymes produced by microorganisms, such as psychrotrophic bacteria, which are typically present in milk. Certain of these enzymes are known to be bound to somatic cells which typically have a least diameter larger than about 0.5 xcexcm. These enzymes can also contribute to the degradation and eventual spoilage of milk.
It is desirable to remove or deactivate the above types of bacteria and enzymes, as well as other microorganisms, in order to prevent degradation of the raw milk and/or resultant milk products.
Typically, microorganisms and enzymes are deactivated by heat treatments and/or removed by filtration of the raw milk. The use of heat treatments alone, however, can impart an undesirable cooked flavor to the milk due to the high temperatures (e.g., 140-150xc2x0 C.) required to deactivate heat resistant microorganisms. Also, filtration alone does not remove all live microorganisms present in the raw milk.
Combinations of filtration and heat treatment have been used to provide a purer milk product wherein more of the microorganisms are removed or deactivated than would occur using either heat treatment or filtration alone. See, for example, WO 98/57549 and U.S. Pat. No. 5,935,632.
Filtration produces a retentate in which the microorganisms are highly concentrated. The concentration of microorganisms in the retentate can be, for example, at least about 10 to 100 times higher than in the raw milk. This retentate typically comprises from at least about 1% to about 10% of the total incoming raw milk. Multiple filtrations of the retentate produce a final retentate in which the microorganisms are even more concentrated, but wherein less milk is retained in the retentate, typically about 1% or less. See, for example, WO 98/57549.
The retentate from filtration can be recycled into the milk or cream processing stream as described, for example, in U.S. Pat. No. 5,935,632, wherein the retentate is added to the cream portion for processing with the cream, or in U.S. Pat. No. 5,683,733, wherein the retentate is added to the milk processing stream before the milk processing stream is fed to the separator. In either case, the number of microorganisms in the cream portion or in the resultant milk product is increased. The retentate can also be discarded as waste, as described, for example, in WO 98/57549. However, this results in a lower yield.
There thus remains a need in the art to develop a process by which the concentration of microorganisms in the milk product can be reduced while the yield or the percentage of raw milk which becomes final milk product is increased.
It is an object of this invention to provide a process and apparatus for producing a sterile milk product that overcomes the deficiencies of the prior processes and apparatus. The process in accordance with a preferred embodiment of the inventions includes: filtering a skim milk portion through at least one first filter to form a first permeate and a first retentate; filtering the first retentate through at least one retentate filter to form a second permeate and a second retentate; and mixing the second permeate into the skim milk portion prior to filtration by the first filter. This invention includes the plant for performing the process.
A second embodiment of the method of this invention includes: separating raw milk to form a skim milk portion and a cream portion; filtering the skim milk portion through a first filter to form a first permeate and a first retentate; filtering the first retentate through at least one retentate filter to form a second permeate; and mixing the second permeate into one or more of the raw milk, the skim milk portion and/or the cream portion. The method may also include steps of heating the first permeate to form a treated skim milk portion, standardizing and heating the cream portion to form sterile cream and remixing the sterile cream with the treated skim milk portion to form a sterile milk product.
A plant for performing the process apparatus in accordance with another preferred embodiment of the invention includes: a conduit for feeding a skim milk portion into a first microfiltration unit for separating the skim milk portion into a first retentate and a first permeate; a conduit for the first retentate; a second microfiltration unit for separating the first retentate into a second retentate and a second permeate; and a conduit for the second permeate. The conduit for the second permeate recirculating the second permeate into the conduit for the skim milk portion.
A second embodiment of the apparatus of this invention includes: a conduit for feeding raw milk into a separating unit; a separating unit for separating the raw milk into a cream portion and a skim milk portion; a conduit for the cream portion; a standardization unit for the cream portion; a heat treatment unit for the standardized cream portion, the heat treatment unit for the cream portion adapted to sterilize the cream portion; a conduit for the skim milk portion; a first microfiltration unit for separating the skim milk portion into a first retentate and a first permeate; a conduit for the first permeate; a conduit for the first retentate; a second microfiltration unit for separating the first retentate into a second retentate and a second permeate; and a conduit for the second permeate. The conduit for the second permeate recirculates the second permeate into one or more of the conduit for the skim milk portion, the conduit for the raw milk, the conduit for the cream portion, and the standardization unit for the cream portion.
Other aspects and features of the invention will be apparent to practitioners in the art upon review of the entire specification and claims.